As the increasing of indoor traffic, indoor coverage will become more and more important for wireless networks. One of the most effective indoor coverage solutions is Distributed Antenna System (DAS). DAS can improve frequency efficiency and enhance system capacity. Since it closes the distance between an User Equipment (UE) and the antenna, DAS can also decrease UE battery power consumption. Currently, some indoor coverage systems adopt fiber-based digital DAS. This kind of DAS is constructed with optical fiber and distributed Remote Radio Heads (RRHs). A RRH may realize all RF front-end functions. The digital baseband signals (I/Q data) are transmitted between the RRH and a base station. Two standardized interfaces, e.g. Open Base Station Architecture (OBSAI) and Common Public Radio Interface (CPRI) protocols can be used for the communication between the base station and its RRHs.
FIG. 1 is a schematic diagram of the basic architecture of a smart digital indoor coverage system. As shown in FIG. 1, the digital indoor coverage system 100 includes a Base Band Unit (BBU), some micro power RRHs (mRRHs) and one or more radio hubs connecting the BBU and the mRRHs. The mRRHs usually have low power, for example, about 100 mW, and their distribution density is high. Whether all mRRHs belong to one cell depends on the user requirement, for example, it depends on whether there is only one cell in a building much of the day. Sometimes, it needs to do the cell split, which means splitting one cell into two or more cells according to different requirements. The radio hub is the routing center. In the BBU, the radio signals from multiple mRRHs are combined to form an uplink signal, while the downlink signals from baseband are distributed to mRRHs.
Ericsson's DOT system and HUAWEI's Lampsite system use Ethernet cable to replace the radio frequency cable for digital signal transmission, and at the remote side, the passive antennas are replaced by mRRHs, and the architecture is almost the same as the one shown in FIG. 1.
In general, outdoor coverage faces relatively open environment, while indoor coverage faces a more complexity and closed environment. A cell's service area often consists of several isolated blocks as separation of concrete wall and floor in the building. In this kind of indoor coverage, it is possible that some UEs in the same cell are isolated enough and they are separated by multiple concrete walls and floors and served by different mRRHs. As the inherent nature of indoor coverage, it is possible to find out a group of UEs in which all UEs are isolated each other such that all UEs in the group can work in the same frequency without any interference, therefore frequency reuse in this kind of group is feasible and system capacity will be improved apparently. Here, this reuse is called as intra-cell frequency reuse due to all resource belonging to one single cell.
For inter-cell frequency reuse, two representative Inter-Cell Interference Coordination (ICIC) techniques are Fractional Frequency Reuse (FFR) and Soft Frequency Reuse (SFR). Both methods split a cell into cell center zone and cell edge zone. In FFR, the frequency spectrum is also divided into two parts: the center band and the edge band. The center band may be used by all cell center users with a reuse factor of one. The edge band may be further partitioned into several sub-bands and reused by cell edge users with a relatively high reuse factor. In SFR, the whole frequency spectrum will be divided into several sub-bands, and one of them will be allocated to cell edge users and the rest of the sub-bands will be used by cell center users with a relatively low power.
Apparently, FFR and SFR solutions do not take full advantage of the isolation and beam accumulation nature of indoor environment, thereby they are more suitable for inter-cell frequency reuse and not particularly suitable for intra-cell frequency reuse.